1. Field of the Invention
The present invention relates generally to satellite-based communications systems and, more specifically to such a system that includes an apparatus and method for reducing latency and buffering associated with multiple access communication.
2. Description of the Prior Art
Conventional satellite-based communication systems maintain an orbit above the earth and contain at least one antenna that provides coverage to an area on the earth""s surface by producing a series of beams that divide the antenna""s coverage area into a pattern of contiguous circular regions or cells. Operationally, an antenna beam is pointed to each cell in a fixed sequential pattern to nominally cover a cellular region where there are multiple users (receivers) distributed within the cell. Several disadvantages are inherent in the conventional system described, the most significant being, latency, buffering, and non-uniform gain distribution.
In conventional systems where time division multiple access (TDMA) downlinks are used with a time framing structure, excess buffering and undesired latency may result. The latency and buffering are necessary because these systems must store continuous or packetized communications in a flexible buffer awaiting the arrival of a correct time slot when the data will be burst communicated to a receiver. Specifically, the satellite beam cycles through the cells on the ground in a fixed sequence and each user""s data is stored in the frame buffer until the satellite beam points to their cell position. For example, in a satellite system servicing cells numbered one through six, the satellite sequentially passes a beam through each of the six cells until all the cells have been serviced after which the service pattern is repeated. Latency and buffering may be particularly evidenced where, for illustration, cell five has just been serviced, the beam moves to service cell six and a packet arrives for a user in cell five. The cell five data must be buffered until the satellite completes the service of cell six and cells one through four.
Likewise, the desire to flexibly support a maximum number of users within a frame period (i.e. a high number of multiple access slots) and a long period for each slot must be balanced against a short frame length. Specifically, because a frame length time period defines the worst case latency, the frame length must be kept short so as to minimize this latency. In systems where no balance is made between frame length and latency, the communications overhead associated with slot transition boundaries and the hardware complexity associated with the slot transition rate is not minimized.
Additionally, the fixed characteristics of a frame based TDMA structure may create constraints on any supported distributions of multiple access capacity. For example, a fixed TDMA framing structure may require excessive re-slotting of individual receivers where there is a high level of unpredictability in the number or receivers serviced. Excessive re-slotting of individual users may also be required where there is data rate variation across a set of receivers, a temporal variation in the data rate per receiver, or unpredictability in the geographical distribution of communications density.
Finally, in conventional systems, there may be a substantial amount of gain variation or signal variation over a cell. As a result, certain users within a cell are disadvantaged; particularly those users at the very edge of the cell where gain is low or the antenna beam may not have optimal pointing.
Based on techniques known in the art for multiple access communications systems, a communications system that reduces the effects of latency and buffering and optimizes the signal gain for ground users is highly desirable.
It is an aspect of the present invention to provide a communications system that includes a unit that includes an antenna disposed on the unit where the antenna has a transmitter and a means for radiating radio frequency (RF) energy along a beam in a plurality of beam coverage areas over a predetermined region of the earth. Each beam has a peak wherein the power of the beam is strongest at the peak and the peak illuminates an area defined by a radius having a predetermined length. The communications system includes a selected user having a terminal station for receiving an RF signal containing packet datum, wherein the terminal station communicates selected user location datum to said unit and receives packet datum from said unit. The communications system further receives, within the unit, an RF signal containing packet datum and transmits the packet datum to a selected user, whereby the antenna beam peak is pointed directly at the selected user. Alternatively, the communications system may comprise a plurality of transmitters where the transmitters simultaneously transmit and are coordinated to decrease co-channel interference.
It is also an aspect of the present invention to provide a method for producing a communications system. The method comprises the steps of providing a unit located at a predetermined location and including an antenna disposed on the satellite, the antenna having a transmitter and radiating radio frequency (RF) energy along a beam in plurality of beam coverage regions over a predetermined region of the earth. Providing a beam having a peak wherein the power of the beam is strongest at the peak, the peak has a radius of predetermined length emanating from a focus of the beam coverage area. Providing a selected user having a terminal station, wherein the terminal station communicates location datum to the unit and receives packet datum from the unit. Receiving at the unit, a RF signal containing packet datum and transmitting, from the unit, a RF signal containing packet datum to the selected user, whereby the antenna beam peak is pointed directly at the selected user.